A system that accurately determines the position of an underwater device would be highly useful for a number of underwater activities. GPS can provide very accurate position location information on the surface of the globe. GPS refers to the Global Positioning System, a constellation of more than two dozen GPS satellites that broadcast precise timing signals by radio to electronic GPS receivers which allow them to accurately determine their location (longitude, latitude, and altitude) in real time. GPS receivers calculate their current position (latitude, longitude, elevation), and the precise time, using the process of trilateration after measuring the distance to at least four satellites by comparing the satellites' coded time signal transmissions. However, since radio waves at the frequency of GPS attenuate very quickly in seawater, the radio-based system cannot be directly used underwater.
Acoustic transmission works very well in water, with low losses, at a sound velocity of approximately 1500 meters/second. As a result, acoustic-based positioning systems have generally been used instead of GPS or other radio-based systems for underwater positioning determination.
One type of prior art system for underwater positioning is known as Long Base Line acoustic positioning (LBL). In most LBL schemes, the Device to Locate (DTL) is active and pings when it receives a sound. A signal sending device sends an acoustic signal to activate the DTL, and sender then receives the response ping and determines the time (round trip delay) to the DTL. The roles can be reversed, but no global timing domain is needed underwater. Where devices are located on the surface of the water, the surface devices can use GPS to get a spatial fix.
An LBL system typically has two elements: the first element includes a number of transponder beacons moored in fixed locations on the seabed (or, for example, on buoys fixed to the sea bed), and the second element consists of an acoustic transducer in a transceiver that is temporarily installed on a vessel or tow fish. The positions of the beacons are described by a coordinate frame fixed to the seabed, and the distances between them form the system baselines. The distance from a transponder beacon to the transceiver is measured by causing the transducer to transmit a short acoustic signal that the transponder detects and then responds to by transmitting an acoustic signal. The time from the transmission of the first signal to the reception of the reply signal is then measured. Since sound travels through water at a known speed, the distance between the transponder beacon and the transducer can then be estimated. The process is repeated for each of the remaining transponder beacons, allowing the position of the vessel relative to the array of beacons to be calculated or estimated.
In principle, navigation can be achieved using just two seabed transponder beacons. In such a case, however, which side of the baseline the vessel is located on may be ambiguous. In addition, the depth or height of the transducer must be assumed (or separately measured accurately). Three transponder beacons is therefore the minimum required for unambiguous navigation in three dimensions and four is the minimum required if redundancy is desired to allow for checks on the quality of navigation. The LBL system works very well, but requires that both the buoys and DTLs transmit data increasing the amount of acoustic bandwidth requires as the number of DTLs are increased, limiting the amount of DTLs that can participate in the scheme. The complexity and power consumption of the DTLs system is increased significantly as it must also transmit data to the buoys.
Another type of prior art system for underwater positioning is known as Short Base Line (SBL) positioning. An SBL system is normally fitted to a vessel, such as a barge, semi-submersible, or a large drilling vessel. A number of acoustic transducers are fitted in a triangle or rectangle on the lower part of the vessel. There are at least three transducers, but the typical number is four. The distance between the transducers (the baselines) are caused to be as large as is practical, typically a minimum of 10 meters. The position of each transducer within a co-ordinate frame fixed to the vessel is determined by conventional survey techniques or from an “as built” survey of the vessel.
SBL systems transmit from one, but receive on all transducers. The result is one distance (or range) measurement and a number of range (or time) differences. The distances from the transducers to an acoustic beacon are measured in a manner similar to what has been described for the LBL system, allowing the position of the beacon to be computed within the vessel co-ordinate frame. If redundant measurements are made, a best estimate can be calculated that is more accurate than a single position calculation. If it is necessary to estimate the position of a vessel in some fixed, or inertial, frame, then at least one beacon must be placed in a fixed position on the seabed and used as a reference point.
With an SBL system, the coordinate frame is fixed to the vessel, which is subject to roll (change in list), pitch (change in trim) and yaw (change in heading) motion. This problem must be compensated for by using additional equipment such as a vertical reference unit (VRU) to measure roll and pitch and a gyrocompass to measure heading. The coordinates of the beacon are then transformed mathematically to remove the effect of these motions. The SBL system suffers the same problems as the LBL system, namely; required underwater bandwidth increases linearly as you add more devices to locate, increased complexity of DTL hardware/software and very difficult setup of the system.
The terms Long Base Line and Short Base Line are used because, in general, the baseline distances are much greater for Long Base Line than for Short Base Line positioning (and even Ultra-Short Base Line positioning; USBL). Because the baselines are much longer, an LBL system is more accurate than SBL and USBL. LBL also has the advantage of positioning the vessel or other object directly in a fixed, or inertial, frame. This eliminates most of the problems associated with vessel motion. In all these systems, the array of seabed beacons needs to be calibrated. There are several techniques available for achieving this, with the most appropriate one being dependent on the task and the available hardware.
There are other prior art systems that provide underwater position determination, including that disclosed in U.S. Pat. No. 5,119,341 to Youngberg, entitled “Underwater GPS System.” The Youngberg scheme is essentially the direct transposition of GPS coding techniques for underwater use, wherein satellites are replaced by buoys and radar waves are replaced by acoustic waves traveling from the buoys to underwater mobiles. The equipment on board the underwater body has an architecture that is very similar to the one encountered in a GPS receiver. A stabilized clock is used for accurate measurement of the time of arrival of the acoustic pulses transmitted sequentially by the buoys. Knowing the velocity of sound in water, it is then possible to calculate the distance to the buoys.
The Youngberg methodology is a full GPS-like scheme, wherein the surface buoys send coded information similar to that sent by the GPS satellites. The device underwater keeps a stabilized clock, compares the arrival time of the start of a message, and uses the time sent data in the message along with the buoy location. A disadvantage of this scheme is that it relies on receiving some very long messages in the noisy underwater environment. The Youngberg system also has the buoys free floating or moving and sends position data on the buoys location on a regular basis. This results in even further complexity in the data message, wherein the time of the start of the data, along with the TIME SENT and LOCATION OF BUOY message data, is required to locate position. It can be very difficult to send high bandwidth acoustic data in the noisy ocean environment, making this approach difficult in practice. In a practical sense the Youngberg scheme requires that the DTLs have a clear data channel from each of the buoys down to the DTLs using an acoustic modem or they can't identify their position, in the underwater acoustic environment this is very difficult to achieve.
The GPS Intelligent Buoys scheme (GIB) is disclosed in U.S. Pat. No. 5,579,285 to Hubert, entitled “Method and device for the monitoring and remote control of unmanned, mobile underwater vehicles.” This system uses a scheme wherein buoys on the surface listen for data sent up from the DTL. Other data is sent back down from the buoys. It is similar to the Youngberg scheme, but instead uses upward acoustic flow of data. The tracking principle is based on measuring the time of arrival at a set of buoys of an acoustic pulse sent by the DTL at a known time. At a regular interval, each buoy transmits to a processing center its D-GPS position and the time of arrival of the acoustic pulses. Knowing the sound velocity, distances from the buoys to the DTL are easily calculated. The minimum number of buoys to deploy is two, as there are only two unknowns, the mobile's depth being sent upwards using a telemetry channel. Some small number of mobiles can be tracked together using time or frequency diversity. The GIB system is limited by the fact that each underwater device to locate (DLT) must use some of the limited underwater acoustic communication channel to send acoustic data to the surface buoys, and therefore the number of devices being tracked is limited to a small amount.